The present invention relates to an improvement in a magnetic recording medium such as a magnetic recording tape or a magnetic recording disc using metallic particles as magnetic particles for the magnetic layer.
The purpose of this invention is to provide a magnetic recording medium having a magnetic layer comprising metallic magnetic particles of high oxidation resistance in a good dispersion state.
Metallic magnetic particles such as metallic iron particles are generally obtainable by reducing metallic oxide particles while heating and, in comparison with metallic oxide magnetic particles, have excellent magnetic performances such as high coercive force (Hc). Thus, the development of a magnetic recording medium using metallic magnetic particles as the magnetic recording element in the future is highly expected. Since, however, metallic magnetic particles are apt to be readily oxidized, they are oxidized with oxygen in the air even in a state being contained in a magnetic layer. With the progress of the oxidation, the magnetic characteristics of the magnetic recording medium such as maximum magnetic flux density (Bs) are lowered, whereby a problem is caused on the stability of the magnetic performances on the storage as well as the use. Further, in comparison with metallic oxide magnetic particles, metallic magnetic particles are apt to cause cohesion on preparation of a magnetic coating composition therewith so that their good dispersion state in the magnetic layer is hardly achieved. Thus, excellent electro-magnetic transducing characteristics such as high sensitivity and high S/N ratio, which are required for a magnetic recording medium, can not be obtained.
In order to overcome the above disadvantages present in the metallic magnetic particles, attempts have been made to improve the oxidation resistance and dispersion state of the magnetic layer by incorporating appropriate additives such as surfactants (e.g. sodium stearate) into the magnetic coating composition. However, these attempts have not produced a satisfactory result.